Passive power management and battery charging for hybrid fuel cell/battery system

ABSTRACT

Disclosed herein is a circuit for passively managing power between a fuel cell stack and a battery in a hybrid system. The circuit includes a buck-boost converter circuit, a direct charge circuit; and a network which interconnects them. The network is configured so that in response to a voltage level in the network being lower than or equal to a maximum battery charge voltage, the battery is charged via the direct charge circuit; and in response to another voltage level in the network which is higher than the maximum battery charge voltage, the battery is charged via the buck-boost converter circuit. Also disclosed is a device incorporating the circuit and a method of passively managing power.

CROSS REFERENCE TO RELATED APPLICATION

The present is a continuation application of pending U.S. patentapplication Ser. No. US 13/185,859 filed on Jul. 19, 2011 and claimspriority therefrom.

FIELD OF THE INVENTION

The present generally concerns electrochemical fuel cells and moreparticularly to passive power management and battery charging for ahybrid fuel cell/battery system.

BACKGROUND OF THE INVENTION

Polymer electrolyte membrane or proton exchange membrane (PEM) fuelcells have intrinsic benefits and a wide range of applications due totheir relatively low operating temperatures (room temperature toapproximately 80° C.). The active portion of a PEM is a membranesandwiched between an anode and a cathode layer. Fuel containinghydrogen is passed over the anode and oxygen (air) is passed over thecathode. The reactants, through the electrolyte (membrane), reactindirectly with each other generating an electrical voltage between thecathode and anode. Typical electrical potentials of PEM cells can rangefrom 0.5 to 0.9 volts; the higher the voltage the greater theelectrochemical efficiency. However, at higher current densities, thecell voltage is lower and there is eventually a peak value in powerdensity for a given set of operating conditions.

Multiple cells are combined by stacking, interconnecting individualcells in electrical series. The voltage generated by the cell stack iseffectively the sum of the individual cell voltages. There are designsthat use multiple cells in parallel or in a combination series, parallelconfiguration. Separator plates (bipolar plates) are inserted betweenthe cells to separate the anode reactant of one cell from the cathodereactant of the next cell. To provide hydrogen to the anode and oxygento the cathode without mixing, a system of fluid distribution and sealsis required.

A number of applications require a wide range of power; for example, theUnmanned Aerial Vehicle (UAV). High powers are required during take-offand climb, with lower average powers for cruise. A typical hybrid fuelcell and battery system would require a DC/DC converter to manage thedifferent voltages from the battery and fuel cell. This is adisadvantage for system weight, volume and efficiency.

With a hybrid fuel cell/battery system, it is also a significantadvantage to charge the batteries while in operation when there isavailable energy from the fuel cell. For this to occur in a safe andreliable manner, the maximum battery charging voltage and current mustbe carefully controlled.

There are several methods to achieve this, but each has its owndisadvantages.

For battery charging, one method is to use a buck DC/DC converter whichwill reduce the stack voltage down to the maximum allowable batterycharge voltage. The issue with this approach is that there are lossesassociated (.about 0.90% efficiency), and therefore the stack voltagemust be significantly higher than the battery charge voltage to enablebattery charging. This means that the battery will be charged only whenthe stack voltage is very high.

Another method is to use a buck-boost DC/DC converter which will supplythe charge voltage at any stack input voltage. The issue with thisapproach is that when the electrical load requires high power levels,energy from the stack will be supplied directly to the bus through thebattery charger, thereby imposing an unnecessary efficiency loss.

Thus, there is a need for improved power management and battery chargingfor a fuel cell/battery system.

SUMMARY OF THE INVENTION

We have discovered that by eliminating a DC/DC converter, a fuel cellstack can be sized to match the voltage/current relationship to that ofthe battery pack, thereby allowing power to be drawn from the fuel cellup to its rated power, and additional power from the battery over andabove this value.

Accordingly, there is provided a circuit for passively managing powerbetween a fuel cell stack and a battery in a hybrid system, the circuitcomprising:

-   -   a) a buck-boost converter circuit;    -   b) a direct charge circuit; and    -   c) a network interconnecting the buck-boost converter, the        direct charge, the network being configured such that:        -   i) in response to a first voltage level in the network being            lower than or equal to a maximum battery charge voltage, the            battery is charged via the direct charge circuit; and        -   ii) in response to a second voltage level in the network            being higher than the maximum battery charge voltage, the            battery is charged via the buck-boost converter circuit.

In one example, the circuit further includes:

-   -   a) a mode selector; and    -   b) a current limit circuit; and        wherein the network interconnects the buck-boost converter, the        direct charge, the mode selector, the current limit circuit and        the network is configured such that:    -   i) in response to the first voltage level in the network being        lower than or equal to the maximum battery charge voltage, the        mode selector connects the direct charge circuit to the battery        so that the battery is charged; and    -   ii) in response to the second voltage level in the network being        higher than the maximum battery charge voltage, the mode        selector connects the buck-boost converter circuit to the        battery so that the battery is charged.

In one example, a voltage comparator controls a switch located in themode selector to passively switch charging of the battery between thebuck-boost converter circuit and the direct charge circuit.

In another example, the current limit circuit is connected to thebuck-boost converter circuit and the direct charge circuit to limitcurrent delivered to the battery.

In another example, the network includes a bus which interconnects thefuel cell stack with the battery. The network includes a first diode anda second diode, the first diode being connected to the fuel cell stack,the second diode being connected between the battery and the bus. Firstand second voltage levels are measured in the bus.

In one example, the battery has low internal resistance.

In another example, the bus has a low voltage limit.

In another example, the circuit further includes an onboard balancingcircuit to monitor individual battery voltages.

In another example, the number of fuel cell stack unit cell active areaand fuel cell number are characterized by a specific overall voltage andcurrent characteristic which matches the battery's overall voltage andcurrent characteristic and cell number, thereby permitting passive powermanagement between the battery and fuel cell stack depending on theresulting combined voltage and current characteristic of the fuel cellstack and battery.

According to another aspect, there is provided a device for passivelymanaging power, the device comprising:

-   -   a) a fuel cell stack;    -   b) a battery; and    -   c) the circuit, as described above.

According to another aspect, there is provided a method of passivelymanaging power between a fuel cell stack and a battery in a hybridsystem, the method comprising:

-   -   i) in response to a first voltage level in a network being lower        than or equal to a maximum battery charge voltage, charging a        battery via a direct charge circuit; and    -   ii) in response to a second voltage level in the network being        higher than the maximum battery charge voltage, charging the        battery via a buck-boost converter circuit.

In one example, the method further includes:

-   -   i) in response to the first voltage level in the network being        lower than or equal to the maximum battery charge voltage,        selectively connecting the battery to the direct charge circuit        so that the battery is charged; and    -   ii) in response to the second voltage level in the network being        higher than the maximum battery charge voltage, selectively        connecting the battery to the buck-boost converter circuit so        that the battery is charged.

In another example, the method further includes limiting currentdelivered to the battery by connecting either the buck-boost convertercircuit or the direct charge circuit to the battery.

In another example, the battery is passively selectively charged.

BRIEF DESCRIPTION OF THE FIGURES

These and other features of that described herein will become moreapparent from the following description in which reference is made tothe appended drawing wherein:

FIG. 1 is a schematic block diagram of a device incorporating a circuitfor passively managing a fuel cell/battery hybrid power system.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, a device for passively managing a fuelcell/hybrid power system is shown generally at 10. The device 10comprises a fuel cell stack 12 that is electrically connected in aparallel configuration to a battery 26. The battery 26 has low internalresistance.

Still referring to FIG. 1, the device 10 includes a passive controlcircuit 30 which is connected between the fuel cell stack 12 and thebattery 26. The circuit 30 comprises a mode selector 16, a buck-boostDC/DC converter circuit 18, a direct charge circuit 20, and a currentlimit circuit 22. The mode selector 16 passively selectivelyelectrically connects either the buck-boost converter circuit 18 or thedirect charge circuit 20 to the battery 26.

A current limit circuit 22 electrically connects the buck-boostconverter 18 or the direct charger 20 to the battery 26. The modeselector 16, the buck-boost converter circuit 18, the direct chargecircuit 20, and the current limit circuit 22 are electricallyinterconnected via a network 32. Internal to the mode selector 16, acomparator circuit compares the input voltage to a predetermined setvoltage level. The buck-boost converter 18 is a switching power supplywhich is capable of converting its output voltage to either lower orhigher that of the input voltage. It is comprised of switchingtransistor, inductor, capacitor, diode and a control circuit. The directcharging circuit 20 is comprised of a switching transistor and someother passive components. The mode selector 16 activates the internalswitching transistor of the direct charging circuit 20 to allow currentflow through this circuit. The current limit circuit 22 is necessary sothat the battery is not overcharged. The current limit circuit iscomprised of a current shunt resistor, switching transistor and a sensecircuit. This circuit senses charge current and when the charge currentis higher than predetermined set current, it throttles current path. Thenetwork 32 includes a bus 34 which interconnects the fuel cell stack 12to the circuit 30 and the battery 26. Two diodes 14 and 24 are connectedrespectively to the fuel cell stack 12 and the battery 26 to protectthem from reverse (back-feed) current and uncontrolled battery charging.The circuit 30 may also include an onboard balancing circuit (not shown)to monitor individual battery voltages.

Load 28 is connected to the fuel cell stack 12 and the battery 26, viathe bus 34 with electrical management to the load 28 being determinedpassively as will be described below.

Referring still to FIG. 1, electrical power management to load 28 fromthe fuel cell stack 12 and the battery 26 is determined passively bymatching the fuel cell stack's 12 voltage and current relationship tothat of the battery 26, thereby allowing power to be drawn from the fuelcell stack 12 up to its rated power, with additional power beingsupplied from the battery 26 at power levels above this value. Thispower management strategy also depends on the battery 26 state-of-charge(SOC), which determines the battery's 26 resulting voltage for a givenelectrical current. The battery 26 is charged by two methods, passiveselection of which depends on the voltage of the fuel cell stack 12,which directly determines the electrical bus 34 voltage when the fuelstack's 12 voltage is higher than the battery 26 voltage. When the bus34 voltage level is lower than the maximum recommended battery chargevoltage, the battery 26 is charged via the direct charge circuit 20,thereby coupling the electrical bus 34 directly to the battery 26. Thebattery 26 charge current is determined by the difference between theelectrical bus 34 voltage and the battery 26 voltage, the battery's 26internal resistance, where the maximum charge current is limited by thecurrent limit circuit 22. If the bus 34 voltage level equals the battery26 voltage, no charging will occur. Conversely, when the bus 34 voltagelevel is higher than the maximum recommended battery 26 charge voltage,the battery 26 is charged via the buck-boost converter charging circuit18, whose output voltage is set to the maximum recommended batterycharge voltage. The maximum charge current is limited via current limitcircuit 22. As previously described, a switch inside the mode selectorcircuit 16 is controlled via a voltage comparator, which passivelyswitches between the direct charge circuit 20 and the buck-boost chargecircuit 18 depending on the bus 34 voltage level. The bus 34 has a lowvoltage limit on the electrical bus load output, thereby reducing outputpower levels when this low voltage limit is reached in order to protectthe stack from overload when the battery state-of-charge (SOC) is low.

By omitting the conventionally used DC/DC converter for power managementto the electrical load 28, we have reduced the overall system mass andcorrespondingly improved system efficiency. Furthermore, we haveincreased the efficiency and safety of battery charging over the fullstack voltage range and have reduced system complexity as a result ofthe passive power management and battery charging circuit. Due to itsdecreased weight, volume and complexity, the device 10 can be used infuel cell/battery hybrid power generation applications, such as forexample, in Unmanned Aerial Vehicle (UAV) applications.

When the fuel cell stack 12 provides a voltage at or below the requiredbattery charging voltage, it is effectively connected directly to thebattery 26. The current limit circuit 22 is integrated into the circuit34 to regulate the maximum current. This allows the fuel cell stack 12to charge the battery 26 at any time when the battery 26 voltage islower than the fuel cell stack 12 voltage, until it reaches the maximumrecommend charging voltage. When the fuel cell stack 12 provides the bus34 with voltage that is higher than the maximum recommended chargingvoltage, the buck-boost DC/DC converter 18 is used to provide therecommended charging voltage. The buck-boost converter 18 eliminates the“dead zone” associated with the buck only charging method. The voltagecomparator is used to switch between the direct and buck-boost chargingmethods. Thus, the direct-buck-boost charging strategy therefore allowsthe battery 26 to be charged over the full stack voltage range when thebattery 26 voltage is below its maximum recommended charging voltage,and when there is available energy from the fuel cell stack 12.

Moreover, the fuel cell stack unit cell is sized in a way (i.e. activearea) to provide the desired current and voltage characteristic of thecell. The number of fuel cells is then chosen to provide the overallvoltage characteristic of the stack. This is then matched with thevoltage and current characteristic, as well as the number of cells, ofthe battery. Therefore, as long as the stack voltage is higher than thatof the battery, power will be supplied from the stack. As increasingpower is drawn, the voltage of the stack drops down to meet the batteryvoltage, and then the power sharing between the stack and batterydepends on the corresponding voltage/current characteristics in thatregion of their operation. Thus, the battery tends to have a “stiffer”voltage which acts as a safety net for the stack, so the stack is notoverloaded.

Operation

Still referring to FIG. 1, the network 32 is configured so that inresponse to a first voltage level in the network 32 which is lower thana maximum battery charge voltage, the battery 26 is charged via thedirect charge circuit 20. Similarly, in response to a second voltagelevel in the network 32 being higher than the maximum battery chargevoltage, the battery 26 is charged via the buck-boost converter circuit18. Also, in response to the first voltage level in the network 32 beinglower than or equal to the maximum battery charge voltage, the modeselector 16 passively connects the battery 26 to the direct chargecircuit 18 so that the battery 26 is charged; and in response to thesecond voltage level in the network 32 being higher than the maximumbattery charge voltage, the mode selector 16 passively connects thebattery 26 to the buck-boost converter circuit 18 so that the battery 26is charged.

Other Embodiments

From the foregoing description, it will be apparent to one of ordinaryskill in the art that variations and modifications may be made to theembodiments described herein to adapt it to various usages andconditions.

What is claimed is:
 1. A device for passively managing the power betweena fuel cell stack and a battery in a hybrid system, the devicecomprising: a) a fuel cell stack having a first voltage and currentcharacteristic; b) a battery having a second voltage and currentcharacteristic; c) a network having a bus, the network electricallyinterconnecting the fuel cell stack, the battery and the bus, the fuelcell stack and the battery being matched to permit passive power sharingbetween the fuel cell stack and the battery depending on the amount ofcurrent being drawn from the hybrid system, so as to share power basedon the resulting voltage in the bus.
 2. The device, according to claim1, in which the network electrically interconnects the fuel cell stack,the battery, and the bus in parallel configuration.
 3. The device,according to claim 1, in which the network includes a buck-boostconverter circuit.
 4. The device, according to claim 3, in which thenetwork includes a direct charge circuit.
 5. The device, according toclaim 4, in which the network includes a mode selector, a current limitcircuit, which network interconnects the buck-boost converter, thedirect charge circuit, the mode selector, the current limit circuit andthe network is configured such that: i) in response to the first voltagelevel in the network being lower than or equal to the maximum batterycharge voltage, the mode selector connects the direct charge circuit tothe battery so that the battery is charged; and ii) in response to thesecond voltage level in the network being higher than the maximumbattery charge voltage, the mode selector connects the buck-boostconverter circuit to the battery so that the battery is charged.
 6. Thedevice, according to claim 5, in which the network is configured suchthat when the first voltage level is higher than or equal to the maximumvoltage charge of the battery, the mode selector passively connects thefuel cell stack and a buck-boost converter circuit to the battery so asto charge the battery.
 7. The device, according to claim 5, in which avoltage comparator controls a switch located in the mode selector topassively switch charging of the battery between the buck-boostconverter circuit and the direct charge circuit.
 8. The device,according to claim 5, in which the current limit circuit is connected tothe buck-boost converter circuit and the direct charge circuit to limitcurrent delivered to the battery.
 9. The device, according to claim 1,in which the network includes a first diode and a second diode, thefirst diode being connected to the fuel cell stack, the second diodebeing connected between the battery and the bus.
 10. The device,according to claim 1, further includes an onboard balancing circuit tomonitor individual battery voltages.
 11. A method for passively managingthe power between a fuel cell stack and a battery in a hybrid system,the method comprising: in a network having a bus, matching a fuel cellstack, having a first voltage and current characteristic, with abattery, having a second voltage and current characteristic, so as topermit passive power sharing between the fuel cell stack and the batterydepending on the amount of current being drawn from the hybrid system,so as to share power based on the resulting voltage in the bus.
 12. Themethod, according to claim 11, comprising: charging the battery chargedvia the direct charge circuit when the bus voltage level is lower thanthe maximum recommended battery charge voltage thereby coupling the busdirectly to the battery.
 13. The method, according to claim 11,comprising: charging the battery via the buck-boost converter chargingcircuit when the bus voltage level is higher than the maximumrecommended battery charge voltage.
 14. The method, according to claim11, in which the fuel cell stack has plurality of unit cells each beingsized so as to provide a desired current and voltage characteristic ofthe cell.
 15. The method, according to claim 14, comprises selecting thenumber of fuel cells to provide the overall voltage characteristic ofthe fuel stack.
 16. The method, according to claim 16, further comprisesmatching the number of fuel cells with the voltage and currentcharacteristic of the battery so that as long as the fuel cell stackvoltage is higher than that of the battery, power will be supplied fromthe fuel cell stack.
 17. The method, according to claim 11, furtherincludes: i) in response to the first voltage level in the network beinglower than or equal to the maximum battery charge voltage, selectivelyconnecting the battery to a direct charge circuit so that the battery ischarged; and ii) in response to the second voltage level in the networkbeing higher than the maximum battery charge voltage, selectivelyconnecting the battery to a buck-boost converter circuit so that thebattery is charged.
 18. The method, according to claim 17, furtherincludes limiting current delivered to the battery by connecting eitherthe buck-boost converter circuit or the direct charge circuit to thebattery.